Erection or maintenance of the following signs may not be permitted in protected areas:
- (a) Signs advertising activities that are illegal under State or Federal laws or regulations in effect at the location of such signs or at the location of such activities.
- (b) Obsolete signs.
- (c) Signs that are not clean and in good repair.
- (d) Signs that are not securely affixed to a substantial structure, and
- (e) Signs that are not consistent with the standards in this part.